The instant invention is directed to an irradiation facility. More specifically, the instant invention is directed to an irradiation facility that slaves accelerator beam current to beam pass conveyor speed. The beam pass conveyor speed is controlled and monitored using a variable frequency drive that is located outside the irradiation cell and does not require speed sensors within the cell.
For the purposes of this application, xe2x80x9ctreating productxe2x80x9d refers to product sterilization, pasteurization, and/or chemical modification.
For the purposes of this application, an xe2x80x9cirradiation facilityxe2x80x9d is a facility that treats product using one or more types of radiation. The most common forms of radiation used to treat product are x-rays, electron beam (e-beam), gamma radiation, and microwave radiation.
For the purposes of this application, a radiation beam is a focused or directed ray of radiation (e.g., an e-beam) or secondary radiation generated from the same (e.g., x-rays generated by aiming an e-beam at a metal target).
For the purposes of this application, an xe2x80x9caccelerator systemxe2x80x9d or xe2x80x9cbeam sourcexe2x80x9d is a device used generate and direct a radiation beam.
For the purposes of this application, a xe2x80x9cbeam pass conveyorxe2x80x9d as a conveyor that transports product through a radiation beam.
For the purposes of this application, a xe2x80x9cradiation shieldingxe2x80x9d is a barrier, or series of barriers, designed to contain radiation emitted by an accelerator system or beam source within a defined area.
For the purposes of this application, a xe2x80x9cradiation cellxe2x80x9d is the area of an irradiation facility within the radiation shielding.
The use of irradiation to sterilize articles, especially medical devices, is known in the art. Ethicon, a Johnson and Johnson Company, located in Somerville, N.J., used e-beams to sterilize medical devices as early as 1956. Furthermore, in 1960, gamma sterilization (using a cobalt-60 source) was used to sterilize medical products at Wantage in the United Kingdom and to inactivate bacillus anthrasis in goat hair in Australia.
Irradiation is now used to sterilize food. One of the earliest irradiation facilities created for this purpose is operated by the Florida Department of Agriculture in Gainesville, Fla. This facility, among other things, uses an e-beam or an x-ray beam to treat products such as blueberries. This facility began operation in the early 1990""s.
Other common uses for irradiation include the chemical modification of polytetrafluoroethylene (PTFE) and silicon wafers. In addition, the use of irradiation to sterilize mail is one of the xe2x80x9cHomeland Securityxe2x80x9d initiatives of the United States government in response to the terrorist attacks of Sep. 11, 2001 and the concurrent spread of anthrax through the mail.
In all irradiation applications, carefully and continuously controlling the dose delivered to product is critical. If a product receives too little radiation, the desired sterilization, pasteurization, and/or chemical modification is not obtained. If a product receives too much radiation, the treatment is, at the very least, inefficient and, at the very worst, damaging to the product. Accordingly, numerous protocols and standards outline the proper procedure for controlling dose. Illustrative standards include: (1) the American Society for Testing and Materials (ASTM) Standard Designation E 1431-91, entitled xe2x80x9cPractice for Dosimetry in Electron and Bremsstrahlung Irradiation Facilities for Food Processingxe2x80x9d (1991); and (2) the Association for the Advancement of Medical Instrumentation/American National Standard/US Department of Defense combined standard entitled xe2x80x9cGuideline for Electron Beam Radiation Sterilization of Medical Devicesxe2x80x9d (1991). A general overview of various standards in the art is included in an article entitled Electron Beam Sterilization,xe2x80x9d written by Marshall R. Cleland and Jeffrey A. Beck and contained in the Encyclopedia of Pharmaceutical Technology, vol. 5, edited by James Swarbrick and James C. Boylan, and published by Marcel Dekker, Inc., New York, N.Y., in 1992. All of these standards, either specifically or generally, express the need to carefully control the critical parameters that affect dose. Critical parameters include, among other things, the selected electron energy spectrum (which affects the depth dose distribution), the average beam current (affects dose rate), the beam pass conveyor speed (affects exposure time), and the beam dispersion parameters, such as beam width.
The beam current and conveyor speed are two of the most important parameters that are monitored, controlled, and coordinated to insure proper dosing. For facilities utilizing continuously-moving conveyors to transport product through an irradiation zone, conveyor speed and beam current control the absorbed dose in the product. If the beam current rises, the dose received by a product passing through the beam increases. If conveyor speed drops, the exposure time and, therefore, dose received, increases. Both actions, therefore, directly impact total dose and must be coordinated.
Traditionally, conveyor speed and beam current have been controlled by a programmable logic controller (xe2x80x9cPLCxe2x80x9d) that slaves mechanisms controlling beam pass conveyor speed to mechanisms controlling accelerator beam current. In other words, any changes in beam current are compensated for by sufficient changes in conveyor speed to insure the desired dosing of product. If the beam current rises unexpectedly (thereby increasing dose), the conveyor speed also rises (thereby decreasing dose), and visa versa, in an amount determined by a pre-set calculation. To date, the mechanisms controlling beam current have always been the master because commercially available accelerators could not respond swiftly enough to conveyor speed changes. For example, a conventional Dynamitron accelerator requires 10 to 20 seconds to change the requested current and a conventional Linac requires from 10 to less than 0.1 seconds depending on the design of the gun and control system to change the requested beam current. In comparison, traditional motors require approximately 0.25 to 0.5 seconds to change the requested conveyor speed.
It would be desirable to create a more responsive system for coordinating conveyor speed and beam current in an irradiation facility. A more responsive system would insure more control and uniformity in dosing.
Traditionally, the actual speed of the beam pass conveyor is measured using a sensor that is physically located near the conveyor within the cell of the irradiation facility. Such devices include tachometers, encoders, and resolvers, and limit switches.
For example, Titan Corporation describes an irradiation facility in U.S. Pat. No. 5,396,074 (xe2x80x9cthe ""074 patentxe2x80x9d). In the ""074 patent, a controller sends a selected speed to a servo motor that drives a beam pass conveyor (called a xe2x80x9cprocess conveyorxe2x80x9d in the ""074 patent). The actual speed of the beam pass conveyor motor is monitored using an encoder. The controller uses a proportional integrated differential (PID) loop to reduce the difference between the selected motor speed and the actual motor speed. The ""074 patent also discusses another system for measuring speed wherein the articles on the conveyor contact limit switches that are feed back to the controller.
These speed measuring devices are expensive. For example, an encoder costs around $2,000.00. Furthermore, because these devices are positioned within the radiation cell and involve a number of parts that are susceptible to radiation damage, the devices have to be carefully monitored for accuracy and the part have to be periodically replaced.
It would be desirable to reduce the number of parts susceptible to radiation damage within the radiation cell of an irradiation facility. This would lower operating costs by reducing the frequency of part replacement.
The instant invention is directed to an irradiation facility wherein the current of a radiation beam is adjusted according to fluctuations in the speed of a beam pass conveyor. This is opposite to the traditional master/slave relationship used in the industry. It is made possible by the introduction of more responsive accelerators that are capable of changing beam current in under 0.1 seconds. When using such accelerators, reversing the traditional master/slave relationship actually speeds up the coordination between beam pass conveyor speed and accelerator current and, thereby, enhances dose control.
The instant invention is also directed to an irradiation facility that measures the speed of the beam pass conveyor without requiring mechanical speed sensors located inside the radiation cell. This is made possible by the introduction of drives that infer motor speed from a distance from changes in induction, resistance, flux current, and/or inertia of a motor. These devices can be positioned outside the radiation cell (i.e., substantially away from the motor on the beam pass conveyor) without hampering their ability to control and monitor the actual speed of the motor. By eliminating more traditional speed measuring sensors that must be positioned proximate to the motor (e.g., encoders resolvers, tachometers, and limits switches) the number of parts within the radiation cell is reduced. Therefore, the number of parts susceptible to radiation damage and the resultant maintenance time and operating costs are minimized.